Added options for CURVE security

- ZMQ_CURVE_PUBLICKEY for clients and servers
- ZMQ_CURVE_SECRETKEY for clients
- ZMQ_CURVE_SERVERKEY for clients
- ZMQ_CURVE_SERVER for servers
- added tools/curve_keygen.c as example
- updated man pages
Clarified man pages on mixed security
Merge pull request #577 from hintjens/master

API for CURVE configuration